Automotive vehicles have a number of electronically controlled actuators which comprise inductive loads supplied by pulse width modulated current. These are, for example, solenoids for hydraulic pressure control in transmissions, motors for throttle positioning, and for fuel pumps which vary fuel line pressure. The average inductive load current is controlled by the duty cycle of the modulator output. Duty cycle is adjusted by changing the width of pulses supplied at a constant frequency or even by changing the frequency of pulses having constant width. In either case, the current through the inductive load is continuous but undulating gradually in response to each pulse as it is turned on or off.
FIG. 1 of the drawings shows a conventional modulator 10 coupled to the high side of an inductive load 12. A vehicle battery 14 supplies power to the modulator 10 containing an output driver 16 comprising a power FET in series with the load 12. The modulator 10 meters current to the load, turning on and off according to the desired duty cycle. When the driver is turned on, a current path I.sub.D includes the battery, the driver and the load. A recirculation diode 18 across the load establishes current path I.sub.R which allows continuation of load current when the driver is off. Thus while the load current is somewhat continuous, the path I.sub.D current flows in pulses and is subject to being turned on and off abruptly. The current discontinuities give rise to radio frequency interference (RFI) and, since the conduction path between the battery 14 and the load may be long enough to serve as an undesired antenna, there is ample opportunity for radiation of the RFI. Such RFI results in broad-band noise which is picked up by the vehicle AM radio and especially effects the lower end of the AM band.
To improve the RFI (noise) condition, a predriver circuit 20 shapes the control signal to the driver 16 in order to shape the turn-on and turn-off waveforms of the I.sub.D current. The predriver 20 includes a waveshaping circuit 22 and an input circuit 24 which is a transistor switch triggered by a pulsed input signal. The junction or node 26 of the input and waveshaping circuits is connected to the control electrode or gate of the driver 16 and the signal developed at the node is thus the control signal of the driver. The waveshaping circuit 22 comprises a resistor 28 and a capacitor 30 in parallel connected between supply line 32 carrying the battery voltage and the node 26. The input circuit 24 comprises a transistor 34 having its base coupled to the input terminal 36 via a resistor 38 and to ground via a resistor 40, its collector connected through a resistor 42 to the node 26 and the emitter is grounded. A Zener diode 44 is connected between the supply line 32 and the node 26 for over-voltage protection of the driver FET's gate input.
The automotive battery voltage is subject to a wide range of variation and the modulator is required to be operative over a wide range of about 7.5 to 26.5 volts. The pulsed input signal causes the transistor 34 to switch between off and saturation. When the transistor 34 is in saturation the steady state control voltage at the node 26 is determined by the battery voltage and the ratio of the resistors 28 and 42. To assure that the gate-to-source voltage V.sub.GS of the FET driver 16 is sufficient at low battery voltage to turn on the driver, the resistor ratio must be large, say, 10:1.
When the transistor 34 turns on, the node 26 is initially at battery voltage and V.sub.GS is zero. The capacitor 30 charges through resistor 42 and transistor 34 to gradually increase V.sub.GS. The FET begins to turn on when a V.sub.GS threshold is attained and passes through an ohmic region for further increases of V.sub.GS to gradually increase conduction of the driver and then the driver reaches saturation. Thus the current in path I.sub.D turns on at a slew rate which is selected by choosing the capacitor 30 and the resistor 42 to yield a fast time constant or charge rate. When the transistor turns off, the capacitor 30 discharges through resistor 28. Since resistor 28 is ten times larger than resistor 42 the time constant will be ten times larger, causing the turn-off event to proceed at a much slower rate. Two consequences of such waveshaping is that there is a time delay between the input pulse rising edge and the onset of FET conduction, and there is a transition time due to the gradual current increase. When the turn-off time constant is short enough to provide acceptable time delays, the turn-on time constant is too short, causing an abrupt current turn-on, and thus generating RFI.
Referring to FIG. 2, an example is shown where the control voltage V.sub.GS rises rapidly to yield a small turn-on delay of the driver current I.sub.D and a steep rising edge of the current which results in current ringing. The turn-on interval is too short to be shown in the drawing. When the control pulse of V.sub.IN terminates, the control voltage V.sub.GS slowly decreases, and after a turn-off delay reaches the transition range of the FET to decrease the current I.sub.D at a desirable slew rate extending over a turn-off interval. Thus the steep current turn-on and the ringing cause RFI; if the time constants are increased to avoid the RFI at the beginning of the pulse, the turn-off delay and interval become very large to substantially stretch the current pulse. This pulse stretching reduces the duty cycle range of the modulator.
It has been proposed to modify the modulator by adding a large capacitor at the power supply input (line 32) to minimize current flow back to the battery transient. This is a partially successful solution although it is expensive. Feedback circuits for altering driver response have also been attempted, also with limited success. Feedback circuits in combination with the large capacitor have also been used. All these modifications are attempts to patch a problem rather then to remove the source of the problem.